1. Field of the Invention
The invention relates to a process for the separation of m-dichlorobenzene and p-dichlorobenzene by extractive rectification using extracting agents and separating off these extraction agents. More specifically, the invention relates to a process for preparing m-dichlorobenzene and p-dichlorobenzene in high purity from mixtures which contain m-dichlorobenzene and p-dichlorobenzene, as well as a process for using certain specified substances as extracting agents in said process.
2. Brief Description of the Prior Art
Pure dichlorobenzene, which are particularly useful as important intermediates for dyes, fragrances and pharmaceuticals and processes for preparing them are generally known in the art. The processes are, however, plagued with difficulties in effecting recovery of pure m-dichlorobenzene and pure p-dichlorobenzene.
In the conventional process for the preparation of dichlorobenzene by the chlorination of benzene, there are obtained chlorobenzene and more highly chlorinated benzenes (e.g. trichlorobenzenes) in addition to the three isomeric dichlorobenzene (DCB). While monochlorobenzene and the more highly chlorinated benzenes can be easily separated from the dichlorobenzene by distillation, the recovery of pure m-dichlorobenzene and pure p-dichlorobenzene is among the most difficult separation tasks of aromatic intermediate chemistry. This is because the boiling points of m- and p-DCB differ by less than 1° C., so that the distillative separation in high purity by fractional rectification is virtually impossible.
The following are illustrative examples of the prior art processes for the separation, and the problems associated therewith. Owing to the difficulty in separation of m-dichlorobenzene and p-dichlorobenzene, the prior art approaches for obtaining the isomers in pure form is via complicated chemical detours. Thus, for example in JP 53044528, the m-dichlorobenzene and the o-dichlorobenzene in a dichlorobenzene mixture are selectively sulphonated with the aid of sulphuric acid. After unreacted p-dichlorobenzene has been separated off, m-dichlorobenzene and o-dichlorobenzene are obtained by desulphonation at higher temperature. U.S. Pat. No. 3,170,961 proposes separation via bromine isomers. In this process, the dichlorobenzene are brominated, and the resulting bromine isomers are separated by distillation in order to obtain pure dichlorobenzene by elimination of bromine. As would be realized, these processes are difficult and expensive.
The separation of mixtures of m-dichlorobenzene and p-dichlorobenzene into pure m-dichlorobenzene and pure p-dichlorobenzene by melt crystallization is not possible because the binary system has a eutectic point at a content of 88% by weight for m-dichlorobenzene. Hence, only one of the two isomers can be obtained in pure form. Moreover, low temperatures (about −30° C.) required for the isolation of m-dichlorobenzene, makes the process uneconomical.
The literature describes other processes for the separation of m-dichlorobenzene and p-dichlorobenzene, which processes are based on the different adsorption on zeolites. JP 11 158 093, for example, mentions separation factors between m-DCB and p-DCB which all technically lead to pure isomers. However, these processes have the disadvantage that the regeneration of the zeolite is complicated and large amounts of solvents have to be circulated.
Of particular interest here is a separation technique comprising extractive rectification. For this technique, there is added to the mixture to be separated an extracting agent which influences the vapour-liquid phase equilibrium via selective interaction in such a manner that separation factors are not equal to one. JP 58 174 333 mentions aniline derivatives as extractive agents. According to this technique, separation factors in the range from 1.08 to 1.16 are obtained. The class of extracting agents mentioned by this patent includes substances which have an unfavourable boiling point, and others which are technically difficult to prepare and are therefore expensive. JP 54 160 322 mentions sulpholane (separation factor 1.15), decanol (separation factor 1.1) and the three isomers of cresol (separation factors 1.07 to 1.14). Owing to its high boiling point and its chemical instability at relatively high temperatures, sulpholane is not suitable for the separation of DCB isomers. In a dissertation (Unverdorben, L., University of Erlangen-Nürnberg, 1992), the separation factors of alkenyl carbonates (ethylene carbonate, propylene carbonate) were determined to be 1.13 to 1.14.
The disadvantages of the abovementioned substances can be summarized as follows. Some of the substances having separation factors of <1.15 require very high separation efficiency; some of the substances are known to be toxic, and some of the substances have boiling points which are so unfavourable that the separation between extracting agent and p-DCB is complicated, and some of the substances exhibit decomposition and as such cannot be used repeatedly.